Stephen Hawking returned to CBS' "Big Bang Theory" on Thursday night for a somewhat surreal cameo: The renowned physicist became locked in an intense game of Words With Friends against the notoriously competitive Sheldon Cooper.

Hilariously, Hawking turned out to be kind of a sore loser, giving up on the game until Sheldon let him win ... and then proving himself to be a sore winner too, by gloating over the victory.

Naturally, Leonard wasn't terribly sympathetic. But the other big news of the episode came from Penny. Our struggling actress/waitress decided to start taking a college class. Watch out, geeks -- there's a new brainiac in town.

"The Big Bang Theory" airs Thursdays at 8 p.m. ET on CBS.

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